Printers and copiers that transfer a toner developed image to a copy sheet have a common problem. All such machines need to accurately register the copy sheet with the image. This can be difficult because the copy sheet and the image may be traveling at different speeds and along different paths. Some have attempted to solve this problem by driving a copy sheet to a fixed gate, but such a technique is generally slow. In modern, high-speed electrographic machines, copy sheets are often registered using opto-electronic systems. Such systems provide added speed and certainty of position.
As a copy sheet approaches its registration position, a suitable registering mechanism straightens the sheet and moves it to its registered position. In order to do this, it is often necessary to axially align the sheet by removing any angular rotation that the sheet may have with respect to the developed image. Once the sheet is axially aligned or otherwise taken out of skew, the sheet must then be jogged into its desired position. A mechanism for removing the skew and for jogging the sheet to its registered position is shown and described in U.S. Pat. No. 5,322,273, whose entire disclosure is herein incorporated by reference.
While the mechanism described above is capable of handling the sheet, the problem remains of how to determine the proper position for the sheet. In at least one prior art system, the problem is solved by using multiple pairs of light emitting diodes and photodetectors. The LEDs and photodiodes are positioned transverse to the sheet's path. The opto-electrical components are mounted on circuit boards that are fixed with respect to the registration mechanism. Multiple pairs of discrete LEDs and photodiodes are used in order to derive edge sensors for different size sheets, such as type letter, legal size, A4 and other sizes. In operation, the registering mechanism first removes the skew from the sheet before the photodiodes and LEDs are operated. The sheet is stopped by the registration mechanism and is moved in one direction and then in the opposite direction until the edge of the sheet just covers or reduces the light received by the photodiode that senses the edge of the sheet. In order words, the sheet is entirely removed from the path of the photodiode and then is incrementally moved back toward the photodiode corresponding to the known sheet length until the edge of the sheet is detected.
Such prior art systems have the advantage of providing certainty of location and are highly reliable. By systematically driving the sheet away from the photodiode and then back towards it until its light is initially attenuated, one can very accurately detect the edge of the sheet. Once the edge of the sheet is detected, the sheet can be jogged to a final reference position. Thereafter, the sheet is released from the registration mechanism and is fed into a transfer station where the toned image is transferred to the suitable registered copy sheet.
Although such registration systems are reliable, they still have a number of drawbacks. They are inherently slow because they must always move the sheet once the sheet is in the registering mechanism. This requires stopping the sheet and jogging the sheet in opposite directions. As productivity demands for electrographic machines increase almost to the level of small printing presses, the time it takes to stop and jog a sheet to register the copy sheet is no longer acceptable. In addition, such systems cannot, without modification, register arbitrary size paper. They depend upon standard size papers for operating a pair of sensors that corresponds to the anticipated size of the paper. If a paper with a non-standard size is used as a copy sheet, the machine cannot accurately register the paper. To do that, the registration system has to be altered to include a further set of sensors designed to register the non-standard paper. However, another non-standard paper size will require still another modification to the machine and another pair of sensors. Therefore, the problem of providing a reliable and fast apparatus for registering copy sheets and for providing a system that can register any size copy sheet remains unsolved.